The long-delayed coroner's inquest examining the disappearance of 15-year-old Arlene Arkinson from Co Tyrone 20 years ago had been due to start next month, but the Police Service of Northern Ireland (PSNI) sought and was granted an indefinite postponement today.

The exact nature of the development in the case was not outlined as a lawyer for the PSNI made the unexpected application before Northern Ireland's senior coroner John Leckey at a preliminary hearing in Belfast but it is understood fresh searches and digs for the teenager's body are set to be undertaken.

Arlene, from Castlederg, went missing in August 1994 after a night out at a disco across the Irish border in Co Donegal.

In 2005 convicted child-killer Robert Howard, who was the last person seen with her, was found not guilty of murder.

The trial jury was unaware of his history of sex attacks and his conviction for murdering south London teenager Hannah Williams in 2001.

The 69-year-old, who is serving a life sentence at HMP Frankland in County Durham, had been due to give evidence via videolink at the inquest, which was scheduled to start on May 12 - almost seven years after it was first ordered.

"Matters related to the criminal investigation have arisen and it's important that hearing this inquest does not trespass on that investigation," he said.

The Arkinson family lawyer Henry Toner QC said the schoolgirl's relatives understood the need for a postponement.

"We would welcome the development and thank the PSNI," he said.

The court was told a further update from police was not expected until September but legal representatives are due to meet again in the summer to discuss issues related to the disclosure of documents linked to the inquest.

In 2011 the police undertook a series of new searches for Arlene's remains but nothing was found.

A year later a flat in Castlederg where Howard used to live was also searched.

The teenager's older sister Kathleen said she understood detectives had recently received new information on the potential location of her body.

"My family is very grateful to the person or the people who gave the information on the whereabouts of where my sister is," said Ms Arkinson, who also thanked her legal team and Mr Leckey.

"It has been 20 long years, she is lying out there in the cold and also she is lying out there longer than she lived."

Ms Arkinson urged anyone else with information to come forward.

"I can't keep going on like this," she said.

"I am asking someone in the public to please help our family to find Arlene.

"We haven't got much money but if we have to stand out in the road and beg for money to find Arlene's remains, we will do it. We have walked through fields before, we have done it through snow, ice, rain, sunshine - we have done everything."

Ms Arkinson claimed police had not acted quickly enough on the new information and had not contacted her personally to inform her of the development.

"I was told this information was significant and, if it is so significant, I want to know why there are delays, why are they not out now picking up Arlene's remains?" she said.

Asked for an update on the investigation, a PSNI spokesman would only say that "inquiries are ongoing".